As the quality of the modern laser color copier increases, the potential for mass production of near perfect counterfeit documents comes closer to becoming a reality. In this regard, it is noted that the Xerox Quick Response Multicolor Printer (QRMP) developed by Xerox for the Department of Defense for highspeed map making applications, uses lasers to produce extremely high resolution color images. Machines of this ilk may be readily accessible in the not too distant future in thousands of offices throughout the country.
The widespread use of such photocopying machines would allow an office worker to make nearly perfect copies of an individual bill of currency. Clearly, this could transform counterfeiting from a crime committed by relatively few and demanding great expertise, into an impulse crime committed by great numbers of office workers.
The severity of this problem is magnified by the fact that the slightly raised print created by the fusing of toner particles gives copied bills a feel which simulates the raised ridges produced by the engraving process used to generate U.S. currency. Thus, the modern laser color copier (which is computer controlled to precisely copy color, contrast and brightness) has the capability of producing bogus currency which both looks and feels like genuine U.S. currency.
Various approaches have been proposed to deter such counterfeiters. For example, it has been suggested that complex, three dimensional plastic holograms be hot-pressed onto U.S. currency. Such holograms would be extremely difficult to copy exactly. Additionally, it has been proposed to dispose minute diffraction gratings into the currency to give the currency a prism-like ability to break white light into the colors of the spectrum. Although such techniques may be effective to forestall counterfeiting, questions have been raised as to whether either of these techniques could endure the wear and tear that bills typically encounter.
The present method and apparatus for forestalling counterfeiting allows a counterfeit document to be identified relatively easily by an average person. In contrast, many prior art counterfeiting thwarting approaches typically require an expert to examine documents with a microscope to identify counterfeit documents.
The present invention employs a fluid jet applicator to record a unique, random pattern on each of a set of documents (e.g., paper currency, corporate or government bonds or any other document or important paper). Focusing on paper currency, if, for example, a bank teller is presented with two allegedly genuine hundred bills which have had the random pattern of the present invention applied thereto, and if both bills are observed as having identical patterns, then at least one is automatically identified a counterfeit.
Thus, according to the present invention, a fluid jet applicator is controlled to produce a truly unique pattern on a set of original documents. Any counterfeiter who merely copies one (or a number) of the genuine documents would be left with a plurality of identical (i.e., non-unique) documents which may then be readily identified by a recipient as not being genuine.
Since any given denomination of paper currency is produced in virtually infinite quantities, it is necessary to utilize a pattern generating technique which produces truly random patterns for the present invention to be optimally effective. It is noted that, although having a generally similar "look" or overall appearance, a careful study of natural wood grain patterns or moire ("watered") silk patterns reveals that such patterns are essentially unique non-repetetive random patterns. The present invention controls a fluid jet applicator to produce such unique patterns and applies such patterns to paper currency or other documents to thwart counterfeiters.
The patterns produced by the present invention may be broadly denoted as "random interference" patterns. Such patterns may include wood grain, moire (watered) silk, waterfall or other related patterns.
Interference patterns (which are generically referred to as "moire" patterns in a May, 1963 Scientific American article, by Oster et al, pages 54-63) typically result from the superposition and resulting interference between two periodic sub-patterns (e.g., two angularly oriented gratings). Such interacting sub-patterns must have transparent interstices regions if they are to be superimposed.
The "moire" silk pattern shown on page 54 of the Oster et al article results from the superposition of two sets of nearly parallel cords to produce a fabric having a shimmering appearance resembling the wavelike reflections on the surface of a pool of water. The interference pattern phenomena is such that a tiny displacement between two nearly aligned arrays of lines will be greatly magnified.
In the present invention, an electrostatic jet applicator is uniquely controlled to selectively create random interference patterns which simulate wood grain, "moire" silk and other related patterns. Initially, the operation of the fluid jet applicator will be generally described followed by a detailed disclosure as to how the random interference patterns are generated and applied to paper currency or other document substrates.
The electrostatic fluid jet applicator of the present invention is designed to apply a fluid (e.g., ink) to a moving document substrate by: (a) selectively charging and recovering some of the fluid droplets continuously ejected from a stationary linear array of orifices affixed transverse to movement of the substrate, while (b) allowing remaining selectively uncharged droplets to strike the substrate (e.g., thereby forming an image on the substrate).
More particularly, fluid is supplied to a linear array of liquid jet orifices in a single orifice array plate disposed to emit parallel liquid streams. These liquid jets break into corresponding parallel lines of droplets falling downwardly toward the surface of a substrate moving transverse to the linear orifice array. A droplet charging electrode is disposed so as to create an electrostatic charging zone in the area where droplets are formed (i.e., from the jet streams passing from the orifice plate). A downstream catching means generates an electrostatic deflection field which deflects all charged droplets into a catcher where they are typically collected, reprocessed and recycled to a fluid supply tank. In this arrangement, only those droplets which happen not to get charged are permitted to continue falling onto the surface of the substrate.
In prior art fluid jet applicators, great care is typically taken to produce droplets that are regularly and precisely spaced, sized, and timed across the orifice array in order to permit proper use of the apparatus. It is well recognized that such uniform droplet production is adversely affected by non-uniform stimulation across the orifice array due to reflected and interfering waves (i.e., so as to produce "standing" waves) such that certain orifices do not have the appropriate stimulation while others have too much.
In accordance with conventional wisdom, prior art fluid jet applicators have been designed to eliminate or reduce such standing wave patterns in order to achieve proper applicator operation. In such applicators, the orifice plates have been limited in length, employed dampening control and many other techniques to eliminate the problems caused by standing wave patterns.
The fluid jet applicator of the present invention utilizes a piezoelectric crystal to artificially stimulate the fluid supply chamber with coherent acoustic energy to purposely generate and exploit the acoustic standing waves therein. As a result, although substantially uniformly sized droplets will be formed at substantially the same frequency from each orifice, individual droplets will be formed so as to be out of phase with adjacent neighbors in accordance with the standing acoustic wave pattern. By selecting only a very short print time, e.g., such that only one or two drops are formed within such a print time and by controlling the frequency of such print time, a wide range of aesthetically appealing, unique, random interference patterns can be created and applied to documents to thwart counterfeiting.